Magnetic-buoyancy-induced Mixing in AGB Stars: Presolar SiC Grains

Abstract

Isotope ratios can be measured in presolar SiC grains from ancient asymptotic giant branch (AGB) stars at permil-level (0.1%) precision. Such precise grain data permit derivation of more stringent constraints and calibrations on mixing efficiency in AGB models than traditional spectroscopic observations. In this paper we compare SiC heavy-element isotope ratios to a new series of FRUITY models that include the effects of mixing triggered by magnetic fields. Based on 2D and 3D simulations available in the literature, we propose a new formulation, upon which the general features of mixing induced by magnetic fields can be derived. The efficiency of such a mixing, on the other hand, relies on physical quantities whose values are poorly constrained. We present here our calibration by comparing our model results with the heavy-element isotope data of presolar SiC grains from AGB stars. We demonstrate that the isotopic compositions of all measured elements (Ni, Sr, Zr, Mo, Ba) can be simultaneously fitted by adopting a single magnetic field configuration in our new FRUITY models.

Magnetic-buoyancy-induced mixing in AGB stars: a theoretical explanation of the non-universal relation of [Y/Mg] to age

“…Recently, Vescovi et al (2020) proposed a new paradigm for mixing induced by magnetic buoyancy in AGB stars. With respect to this work, we expanded the metallicity range of our theoretical investigation and highlighted a new relationship between metallicity and high-mass element yields.…”

“…FRUITY models are calculated by applying a simple exponentially decreasing profile of the convective velocities at the inner border of the convective envelope (Cristallo et al 2009). Recently, these models have been revised by considering the mixing triggered by magnetic fields (Vescovi et al 2020). This led to the formation of a more extended and flatter 13 C pocket with a lower 13 C concentration.…”

“…This led to the formation of a more extended and flatter 13 C pocket with a lower 13 C concentration. We refer to Vescovi et al (2020) for the details of this implementation. New magnetic models better reproduce presolar grain experimental data by matching almost all the isotopic ratios.…”

“…New magnetic models better reproduce presolar grain experimental data by matching almost all the isotopic ratios. Vescovi et al (2020) presented three models with the same initial mass (M = 2 M ) and different metallicities (Z = 10 −2 , Z = Z , and Z = 2 × 10 −2 ). To better identify the yttrium trend as a function of the metallicity, we extended the 2 M set by calculating three additional models (Z = 3 × 10 −3 , Z = 6 × 10 −3 , and Z = 3 × 10 −2 ).…”

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

“…[20] suggested that the formation of the 13 C-pocket can be induced by magneticbuoyancy (based on the formalism by [21]). This last hypothesis was then tested by [22], who demonstrated that a wide range of isotopic ratios, measured in SiC presolar grains, can be simultaneously reproduced by adopting a single magnetic field configuration. However, there are some drawbacks affecting the robustness of this scenario, which warrant more work and discussion.…”